As is known, proper operation of numerous devices is dependent upon precise regulation of the operating temperature. This may involve difficulties, especially when the devices, for optimizing performances or simply for reducing the overall dimensions, must be integrated on a single chip of semiconductor material also comprising devices that dissipate high powers. In this case, in fact, the problem arises of thermally insulating the regions in which the power devices are formed, which are at a high temperature, from the regions that must be kept at a controlled temperature.
For example, the treatment of some fluids involves an increasingly precise temperature regulation, in particular when chemical or biochemical reactions are involved. In addition, frequently the need is felt to use very small amounts of fluid since the fluid is costly and not always readily available.
This is, for example, the case of the process of DNA amplification (polymerase chain reaction process, or PCR process) in which the following are important for obtaining good reaction efficiency or even for obtaining the reaction itself: precise temperature control in the various phases (repeated preset thermal cycles are required); the need to avoid as far as possible thermal gradients where the fluid is made to react (so that in these areas there may be a uniform temperature); and also the quantity of fluid used (which is very costly).
Other examples of treatment of fluids having the characteristics indicated above are, for instance, linked to the performance of chemical and/or pharmacological analyses, biological tests, etc.
At present, various techniques exist that enable thermal control of chemical or biochemical reagents. A first technique uses a reactor comprising a glass or plastic base on which a biological fluid is deposited by means of a pipette. The base rests on a hot-plate called “thermo-chuck”, which is controlled by external instrumentation.
Another known reactor comprises a heater, controlled by appropriate instrumentation and on which a biological fluid to be examined is deposited. The heater is supported by a base which also carries a sensor that is arranged in the immediate vicinity of the heater and is also connected to the instrumentation for temperature regulation, so as to enable precise temperature control.
Both types of reactors are often enclosed in a protective casing.
A common disadvantage of the known reactors described lies in the large thermal mass of the system; consequently, they are slow and have high power absorption. For example, in the case of the PCR process mentioned above, times of about 6-8 hours are required.
Another disadvantage of known solutions is that, given the macroscopic dimensions of the reactors, they are able to treat only relatively high volumes of fluids (i.e., minimum volumes of the order of milliliters).
The disadvantages referred to above result in very high treatment costs (in the case of the aforementioned PCR process, the cost can amount to several hundreds of dollars); in addition, they restrict the range of application of known reactors to test laboratories alone.
In order to overcome the above-mentioned drawbacks, starting from the late eighties miniaturized devices have been developed, and hence ones of reduced thermal mass, which are able to reduce the times required for completing the DNA-amplification process.
The first of these devices is described in the article by M. A. Northrup, M. T. Ching, R. M. White, and R. T. Watson, “DNA amplification with a microfabricated reaction chamber”, Proc. 1993 IEET Int. Conf. Solid-State Sens. Actuators, pp. 924-926, 1993, and comprises a cavity formed in a substrate of monocrystalline silicon by anisotropic etching. The bottom of the cavity comprises a thin silicon-nitride membrane, on the outer edge of which heaters of polycrystalline silicon are present. The top part of the cavity is sealed with a glass layer. Thanks to its small thermal mass, this structure can be heated at a rate of 15° C./sec., with cycle times of 1 minute. With this device it is possible to carry out, for a volume of fluid of 50 μl, twenty amplification cycles in periods approximately four times shorter than those required by conventional thermocyclers and with a considerably lower power consumption.
However, the process described (as others currently used based upon bonding two silicon substrates pre-formed by anisotropic etches in KOH, TMAH, or other chemical solutions) is costly, has high critical aspects and low productivity, and is not altogether compatible with the usual steps of fabrication applied in microelectronics.
Other more recent solutions envisage forming, inside a first wafer of semiconductor material, buried channels connected to the surface via inlet and outlet trenches, and of reservoirs formed in a second wafer of semiconductor material by anisotropic etching, and bonding together the two wafers.
This solution, however, is also disadvantageous in that the process is costly, critical, has low productivity, and requires the use of a glass paste containing lead (so-called “glass frit”) for bonding the two wafers together. The problem of thermal insulation may also regard sensors or actuators comprising micro-electromechanical systems (MEMS), which sometimes must be integrated with power devices. In these cases, insulation is required both in order to prevent subjecting the micro-electromechanical structures to dangerous thermal stresses, and because the efficiency and precision of the devices are linked to the presence of well-determined operating conditions.